Light olefin products (e.g., ethylene, propylene, and butene) generated by various technologies, such as gas to olefins, methanol to olefins, steam cracking or fluid catalytic cracking contain highly unsaturated byproducts, such as alkynes and alkadienes. These byproducts are subsequently removed from light olefins, because they can be poisons to downstream olefin polymerization catalysts.
One process for removing (via conversion) unsaturated byproducts, such as alkynes, such as acetylene and methyl acetylene, and alkadienes, such as propadiene and butadiene, from light olefin streams is selective hydrogenation. Selective hydrogenation is used to partially saturate the alkynes and butadienes to form the desired olefins. This process has been carried out using a variety of catalysts. Examples of selective hydrogenation catalysts include nickel or palladium and their alloys supported on alumina.
To perform the selective hydrogenation, four unit types are typically used: (i) front-end selective catalytic hydrogenation converters, (ii) back-end selective catalytic hydrogenation converters, (iii) methyl acetylene/propadiene (MAPD) selective catalytic hydrogenation converters and (iv) butadiene (BD) selective catalytic hydrogenation converters. These converters typically involve different feeds based on the specific process.
Typical acetylene conversion processes utilize fixed bed tubular converters incorporating engineered catalyst structures to manage heat and mass transfer within the converter. The engineered catalyst particles may be impregnated with active catalyst to convert feeds (e.g., acetylene) into products (e.g., ethylene). These processes tend to be utilized with lower temperature pyrolysis processes, such as steam cracking, which produce ethylene along with other lower amounts of byproducts, such as acetylene. As an example, the acetylene processed in a steam cracking process is typically less than (<) 2 mole percent (mol %) and more typically <0.3 mol %.
With higher acetylene concentrations, U.S. Pat. No. 4,705,906, describes a process that utilizes greater than (>) 1 mol % carbon monoxide in its process. The catalyst comprises a metal oxide or sulfide or mixture of metal oxides or sulfides having hydrogenation activity, for example ZnO either alone or in combination with other metal oxides or sulfides. As other examples, U.S. Pat. No. 7,153,807 discloses a selective hydrogenation process that uses non-palladium catalyst as the selective hydrogenation catalyst.
As an enhancement to the process, additional equipment has been proposed for hydrogenating hydrocarbons, such as a microchannel converter. As an example, U.S. Pat. No. 7,404,936 discloses that microchannel converters can be used in a variety of chemical reactions including hydrogenation. Palladium is given as one of many types of catalysts that can be used in the process.
Despite a long history of improvements to the selective hydrogenation processes and systems that can be used in the hydrogenation of hydrocarbons, additional problems remain. Such problems include the production of significant amounts of undesirable compounds, such as saturates (e.g., ethane, propane, butane), as well as the production of green oil (C4+ oligomer compounds). These saturates are typically formed due to over-hydrogenation of the alkynes and/or alkadienes and the non-selective hydrogenation of olefins. Green oil is generally formed as a result of oligomerization of the alkynes and/or alkadienes and/or olefins. Both saturates and green oil are undesirable due to a loss of the desired mono-olefins component of the product stream. Green oil is additionally troublesome in that it further decreases catalyst life by depositing heavy compounds on the catalyst surface.
In addition, other limitations are associated with the removal of heat and recovery of usable heat from the exothermic reactions in hydrogenation. For instance, as the process involves exothermic reactions, the process may lose control of the reactions (e.g., temperature control) if the temperature within the unit is not properly managed. For streams with low levels of acetylene (e.g., <2 mol %), the reactions may be managed selectively using conventional techniques because of the lower catalyst activity. However, for streams containing higher levels of acetylene (e.g., ≧2 mol %), conventional processes have problems controlling the reaction temperatures, while still remaining highly selective. In addition, the conventional processes are limited by heat and/or mass transfer. As a result, the catalyst has to be configured with low metal loadings to lower catalytic activity, may utilize readily accessible surface area and may include catalyst inhibitors (e.g., carbon monoxide). That is, as the process does not efficiently remove heat, the process has to limit reactions to prevent overheating of the unit. As such, the conventional processes are limited by heat generation and fail to effectively recover energy from the process.
As yet another problem, the selectivity is typically modest for vapor phase processes with a portion of the acetylene and/or ethylene converting to ethane and/or other undesired products. This low selectivity may not be problematic for lower temperature conversion processes (e.g., steam cracking), which involve streams having a relatively low acetylene content. However, for higher acetylene content streams, the lower selectivity results in recycles and/or multiple conversion stages. These inefficiencies increase the cost and size of equipment and operations and add undesired complexity to the system.
Accordingly, enhancements in selective hydrogenation processes are desired to increase the hydrogenation of alkynyl-containing compounds and polyunsaturated compounds over hydrogenation of mono-unsaturated compounds. Enhancements in selective hydrogenation processes are also desired to enhance efficiency in the process, such as increasing heat recovery from reactions in the process and increasing feed conversion rates. Further, it may also be desirable to integrate the hydrogenation process with the conversion of hydrocarbons to provide additional efficiencies for the system.